The present application relates to pitot tube separators for mechanically mixed fluids and solids, and to such separators and systems especially useful in petroleum recovery from wells where produced oil is the working fluid in down-in-the-well pumps.
Production oil from petroleum wells has been used as the power fluid of petroleum pumps located in the wells in petroleum recovery zones. The oil is from a production fluid typically of oil, gas, water and solids. Before the oil of this fluid can be used as the power fluid, the water, gas and solids must be removed.
"Three-phase" centrifugal separators separate production fluid into the phases of solid, water, oil and gas. The separated oil is of adequate purity to serve as the power fluid for the petroleum pumps. Known three-phase centrifugal separators include a rotor driven in rotation in a housing. A chamber in the rotor receives input fluid and subjects that fluid to centrifugal force. The differences in the constituent fluid densities stratifies the fluid into zones with the heaviest fluid being the furthest from the axis of rotation of the rotor and the lightest at the axis of rotation. The solid constituents also separate in accordance with their density, but a practice has been to take the separated solid off with a carrier liquid. The carrier liquid is taken from one of the liquid phases and may be water, for example. After the zone at the axis, the remaining constituent zones are annular, doughnut shaped. Appropriate taps draw the separated phases from the rotor.
Pitot tube pumps with centrifugal separators use a pitot tube in the chamber of a rotating casing to intercept and draw off liquid. Interception is at a fixed radial zone relative to the axis of rotation of the casing. Solids pass through nozzles opening at the radial perimeter of the casing. U.S. Pat. No. 3,817,446 describes such a pump and separator.
The relative amounts of each of the phases of production fluid can vary. The tap for the oil phase will draw off gaseous phase fluid if the water phase in the rotor drops too much and permits the oil phase to move radially outward in the rotor and away from the oil tap. This is undesirable. If the fraction of water phase drops off and the fraction of oil increases, oil can be the only phase seen by the oil tap. Too much oil, however, can result in oil coming off the water tap in an exceptional case. The increase in oil can be at the expense of gas, in which case the increased oil fraction can exit a rotor through the gas tap and provisions must be made for this possibility.
U.S. Pat. No. 3,960,319 describes a multiple phase separator and a technique for assuring the maintenance of the distinct zonal phases in the separator. The separator has a rotor for centrifuging and a pitot tube tap for taking off an oil phase. The technique maintains the pressure in a water zone of the separator and supplies makeup water to this zone in the event that the pressure drops, and permits loss of water from the zone when the pressure increases. This maintains a constant radial position of a water-oil interface and the oil behind the interface within a radial zone that includes the oil tap.